Internal combustion engines generate exhaust gases during combustion. The exhaust gases often contain contaminants that, when left untreated, can harm the environment. The contaminants include oxides of nitrogen (“NOX”), which can react with atmospheric air to form smog, ozone, and acid rain. Although there are many different oxides of nitrogen (e.g., nitrogen monoxide, nitrogen dioxide, nitrous oxide, nitroslyazide, nitrate radical, etc.) used herein, NOX primarily refers to nitrogen monoxide and nitrogen dioxide. Oxides of nitrogen other than nitrogen monoxide and nitrogen dioxide are not generally regulated by internal combustion engine emissions standards. Many regulatory territories (e.g., the United States, the European Union, India, China, etc.) regulate the amount of NOX that is permitted to vent into the ambient environment from internal combustion engines. Accordingly, exhaust aftertreatment systems are employed. For example, some internal combustion engines utilize a selective catalytic reduction (“SCR”) system that injects diesel exhaust fluid (“DEF”) into the exhaust stream. The DEF is a liquid that typically includes urea. When the urea is heated by the exhaust gases, the urea breaks down into ammonia, which reacts with NOX to form water and nitrogen thereby reducing NOX emissions.
Many SCR systems inject a variable dose of DEF into the exhaust stream (e.g., into an exhaust component carrying exhaust gases from the internal combustion engine) at constant time intervals. In such systems, the amount of DEF dosed into the exhaust stream is varied depending on the amount of NOX in the exhaust stream. For example, at engine idle speeds, a first amount of DEF may be dosed into the exhaust stream at a given time interval, and during higher than idle engine operating speeds, a second amount of DEF, which is larger than the first amount, may be dosed into the exhaust stream at the same given time interval. The fixed interval between successive doses may occur every millisecond.
However, such fixed interval, variable amount SCR systems have numerous drawbacks. Variable volume injectors can suffer from inaccurate doses of DEF (e.g., 20-30% error in the amount of DEF actually dosed vs. the amount of DEF intended to be dosed)—particularly at low volume doses of DEF. Further, under certain conditions, particularly at lower than normal exhaust gas temperatures (e.g., less than 200 degrees Celsius), excess DEF can form deposits or film on the various exhaust gas components. In some situations, deposits or film can form under normal operating conditions at locations where continual dosing can lower local temperatures on impingement surfaces even though there is adequate heat in the exhaust stream to decompose the DEF. Fixed interval, variable amount SCR systems can also require sophisticated controllers, high cost DEF pumps, and more complicated and less robust injectors.
Further, fixed interval, variable amount SCR systems often utilize a single reductant injector. The single injector continuously sprays the reductant into a component of the aftertreatment system, where the reductant is at least partially broken down into ammonia by the heat of the exhaust gases and the heat of the component of the aftertreatment system. However, the continuous spraying of the reductant into the component may result in localized cold spots on the wall of the component where the reductant contacts the component. The localized cold spots may inhibit the breakdown of the reductant into ammonia. Accordingly, reductant deposits and films can form in the areas of the cold spots of the component. Such deposits and films may be mitigated through heating of the component with a secondary heating device or through modulation of the quantity of reductant injected. However, these solutions require auxiliary emission control devices that can drive up the cost and complexity of the aftertreatment system.